This invention relates to a method of making a piston for a machine such as an internal combustion engine and more particularly to a method for manufacturing a composite blank and a method for forging a composite piston.
The piston in a reciprocating machine is obviously a very critical part of the mechanism. This is particularly true with internal combustion engines in as much as the piston is the part of the engine that receives the explosive force from the combustion and transmits it through the connecting rod to a crankshaft for providing an output force. The various parts of the piston have specific functions in order to achieve this result
The head of the piston must be able to withstand the compressive force and temperature of combustion. Also the area adjacent the upper surface of the head forms a ring groove area where the piston rings are supported. These provide a sealing function with the cylinder bore so as to confine the combustion products.
The piston is also provided with pin bosses that receive the piston pin and which transmit the force from the piston to the connecting rod through the piston pin. Obviously, there are high forced transmissions in this area
In addition, the piston as a skirt portion that rubs against the cylinder bore and which assists in maintaining the piston in an upright condition within the cylinder bore. In addition, the side thrusts on the piston are taken by the skirt and thus it is also subjected to forces and must have high abrasion resistance due to its rubbing action with the cylinder bore.
Thus, it should be apparent that the different parts of the piston have different functions that require optimally different materials. Of course, it is possible to form the entire piston from the same material but this can give rise to high costs and also high weights. It is important to reduce the weight of the piston so as to reduce the inertial loading on the engine and provide high power outputs and high engine crankshafts speeds. Also, the lighter the weight the lighter the balancing masses in the engine can be in order to reduce vibrations.
Some of these functions can be achieved by changing the dimensions of the piston either alone or in combustion with changing the materials. For example, the sealing function can be improved if the piston ring area is made greater and a greater number or greater size of piston rings is employed. However, this causes emission problems in that the area around the piston rings may retain combustion products and can cause some emission concerns.
Thus, there has been proposed the formation of pistons with different materials, each serving its intended purpose for the particular part of the piston in which it is positioned. However, this is quite a difficulty in adhering or connecting these different materials to each other to provide a unitary structure. Some more methods of connections can be employ brazing or welding. However, when applied with these additional heats in order to connect the materials together, then deterioration in the properties of the associated and affected materials can result thus defeating the main purpose of the composite construction.
It has also been proposed to improve the strength of the piston in certain areas by casting in inserts in the areas where stresses is highest. For example, it has been proposed to cast in inserts in the area of the piston pin bosses so as to increase their strength without adding significantly to the overall weight of the piston. However, this also has some of the same problems aforenoted in connection with using dissimilar materials. Furthermore, the casting process becomes somewhat complicated and thus this method does not totally solve the problem.
Forging is another technique by which composite materials may be used. Some methods have been proposed, but they have not been totally successful in achieving the desired bonding strength. Therefore there has been proposed a method and construction that employs a combination of powdered metal technology and forging bonding that can produce excellent results. This is disclosed in the co-pending application of certain of the inventors hereof entitled xe2x80x9cPiston For Internal Combustion Engine And Process Of Making Samexe2x80x9d, Ser. No. 08/859536, Filed May 20, 1997 and assigned to the assignee hereof.
The materials utilized also are important not only to achieve the desired properties, but also the proper bond. Basically, pistons for engines are generally formed from aluminum or aluminum alloy materials. The aluminum has the advantage of light weight and relatively high strength. However, the use of alloy materials has been resorted to so as to improve certain characteristics.
For example, silicon (Si) in alloyed with the aluminum to increase abrasion resistance and resistance to hardening under temperature. Copper (Cu) and Magnesium (Mg) have also been employed for increasing strength. At times, however, these alloying elements can present some problems in that their inclusion in a casting process can cause a difference in particle sizes which can offset some of the benefits of the alloying.
There has also been proposed, a method of forming a piston material by a form of sintering process that permits the forging of a piston to obtain the desired characteristics. Such an arrangement is disclosed in the co-pending application entitled xe2x80x9cPiston For Internal Combustion Engine And Material Thereforexe2x80x9d, Ser. No. 09/022647, filed, Feb. 12, 1998, and also assigned to the assignee hereof.
In accordance with the features hereof these materials are combined with lower costs materials to form a composite piston that will provide the performance desired along with lightweight and lower costs.
It is, therefore, a principal object to this invention to provide a method of forming an improved piston construction for an internal combustion engine.
It is a further object to this invention to provide an improved method by which a lightweight, high strength and high abrasion resistant, composite piston for a reciprocating machine may be formed.
It is a further object to this invention to provide an improved method for making a low cost piston having the desired material requirements in the various areas of the piston.
It is a further object to this invention to provide an improved method for manufacturing a composite piston of the aforenoted type.
In accordance with the methods described in the aforenoted co-pending applications, a composite blank from which the piston will be forged is manufactured. The blank consists of a pair of cylindrical blank components that are placed together in a forging die and from which the finished composite piston is forged. It is important m connection with the forging process to ensure a good bond between the two dissimilar metal.
In accordance with a further feature of this invention, it is an object to provide a blank from which a piston can be forged from dissimilar materials wherein an initial bonding between the materials is created when the blank is formed and before it is forged into the piston shape.
This invention is adapted to be embodied in a method for forming a blank form dissimilar materials from which a piston may be forged. A first material is extruded under pressure into a generally cylindrical object in a chamber wherein a powder of the other material is contained. The powder and the formed cylindrical object are passed through an extrusion die under sufficient pressure so as to solidify the powder and establish a bond between the outer surface of the first material and the surrounding other material.